Fiber optic diaphragm sensors for engine knock and misfire detection

Abstract

The modulation depth of a fiber optic diaphragm sensor is increased by tapering the tip end of the optical fiber or fiber bundle. In a second embodiment, the modulation depth is increased by mechanically leveraging the diaphragm movement relative to the tip end of the optical fiber or fiber bundle. In either embodiment, the operating temperature of the tip end of the optical fiber or fiber bundle can be reduced relative to the operating temperature of the diaphragm. Thus, the diaphragm sensor can be directly exposed to combustion temperatures in an internal combustion engine and advantageously the sensor can be embodied in a spark plug adjacent the electrodes.

I claim: 
     
       1. In a fiber optic diaphragm sensor comprising a body having a cavity approximately 3 mm across therein, a diaphragm attached to the body and partially enclosing the cavity, the diaphragm being moveable in response to changes in fluid pressure exerted upon the diaphragm, and an optical fiber inserted in the body with the tip end of the optical fiber disposed to inject a light beam into the cavity and receive a reflected light beam,   the improvement comprising moveable reflective means interposed in the cavity to intercept and reflect back the injected light beam, mechanical means interconnecting the reflective means to the diaphragm whereby movement of the diaphragm in response to changes in fluid pressure activates responsive movement of the reflective means and modulation of the light beam reflected back to the tip end of the optical fiber, said mechanical means interposed between the diaphragm and reflective means to provide a substantial temperature differential between the diaphragm and reflective means when the diaphragm is hot.   
     
     
       2. The fiber optic diaphragm sensor of claim 1 wherein the interconnecting mechanical means leverages movement of the reflective means to magnify the movement of the diaphragm. 
     
     
       3. The fiber optic diaphragm sensor of claim 1 wherein the interconnecting mechanical means is connected to the diaphragm substantially at the location of maximum diaphragm amplitude and to the reflective means at a location substantially spaced from the location of maximum amplitude of the reflective means. 
     
     
       4. The fiber optic diaphragm sensor of claim 1 wherein the reflective means is attached to the body by a fulcrum and the interconnecting mechanical means is connected to the reflective means at a location between the fulcrum and the location at which the light beam is centered on the reflective means. 
     
     
       5. A spark plug and sensor combination comprising a ceramic core and first electrode extending therefrom, a sleeve surrounding the core and a second electrode extending from the sleeve, a small bore in the spark plug adjacent the electrodes and a sensor located in the small bore, the sensor comprising a body having a cavity therein, a diaphragm attached to the body and partially enclosing the cavity, the diaphragm being moveable in response to changes in rigid pressure exerted upon the diaphragm,   optical means to inject a light beam into the cavity and receive a reflected light beam,   moveable reflective means interposed in the cavity to intercept and reflect back the injected light beam, mechanical means interconnecting the reflective means to the diaphragm to cause movement of the reflective means in concert with movement of the diaphragm, said mechanical means interposed to provide a substantial temperature differential between the diaphragm and reflective means when the diaphragm is hot.   
     
     
       6. The spark plug and sensor combination of claim 5 wherein the interconnecting mechanical means leverages movement of the reflective means to magnify the movement of the diaphragm. 
     
     
       7. The spark plug and sensor combination of claim 5 wherein the ceramic core and first electrode are offset from the center of the spark plug and the small bore is located in the sleeve.